Split out the command handling for split debug info, we're going
to want to propagate some information through the module into
the back end and so need to pass it through to codegen.

Also make the methods file static so we can use them in other places.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175916 91177308-0d34-0410-b5e6-96231b3b80d8